(1) Field of the Invention
The present invention relates to cell units for solid oxide fuel cells and power generators using such cell units.
(2) Related Art Statement
Recently, the fuel cells have attracted public attention as power generators. Since fuel cell is a device which directly converts chemical energy possessed by the fuel to electric energy and which is free from restraint of the Carnot cycle, the fuel cell has an essentially high energy-converting efficiency. In addition, the fuel cell can use a variety of fuels (naphtha, natural gas, methanol, coal-reformed gas, heavy oil, etc.), and has less public nuisance, and its power-generating efficiency is not influenced by the scale of the plant. Accordingly, the fuel cell is an extremely promising technique.
Particularly, since the solid oxide fuel cell (SOFC) functions at high temperatures of around 1,000.degree. C., the reaction on the electrodes is extremely high, no catalyst of an expensive noble metal such as platinum is necessary at all, the polarization is small, and the output voltage is relatively high. Thus, the energy-conversion efficiency is conspicuously high as compared with other fuel cells. In addition, since the constituent parts are all constituted by solid material, the SOFC has high stability and long use life.
As mentioned above, since the constituent parts of the SOFC are all solid, SOFCs possessing various structures have been proposed. These SOFCs are broadly classified into planar type and cylindrical type (General Energy Engineerings, 13-2, 1990). Since the electromotive voltage of a cell unit in these SOFCs is about 1 V in an open circuit, and the current density is about several hundreds mA at the maximum, it is important that the cell units, each having a large power-generating area, can be easily connected in parallel and in series in the practical application. From this standpoint of view, the structure of the cell units and their stack (collected cell) must be examined.
However, in the case of the planar cell, it was difficult to produce the cell units having high plane accuracy and large area due to brittleness of ceramics. In order to solve this problem, a method is proposed to interpose a soft material between cell units for electrically connecting the cell units (Japanese patent application Laid open No. 3-55,764). However, the size of the ceramic flat plates to be integrally produced has a limit even in this case, and the structure is complicated. Thus, it is difficult to connect the cell units in parallel, so that it is difficult to increase the intensity of the output current. Furthermore, it is difficult to gas-tightly seal the planar cell units at their ends.
To the contrary, Westinghouse Electric Corporation proposed a currently most advanced production technique of cylindrical cells in which a cylindrical type fuel cell having structurally high strength is employed to mitigate brittleness possessed by ceramics and one end of the cell unit is closed to attain a sealed structure (General Energy Engineerings, 13-2, 1990). Further, this technique is excellent in that the cell units can be easily connected in parallel or in series.
However, since current flows in parallel to the solid electrolyte film in this structure, the current-flowing path is long and power is consumed in this flowing step. In order to solve such a problem, a method is proposed, in which a current-flowing path is provided at a location other than a circumferential direction of the section of the cylinder (Japanese patent application Laid-open No. 63-261,678). However, even in this case, the internal resistance of the cell cannot be decreased to such a degree as in the case that current is passed vertically to the solid electrolyte film. Further, although the above method requires that a gas-tight solid electrolyte film is formed on the porous cylindrical support, it is necessary to use a high technique having low production speed and high production cost, for example, EVD.
On the other hand, since the SOFC is operated at high temperatures of around 1,000.degree. C., the SOFC has an advantage that a cogeneration system may be attained by utilizing waste heat of the SOFC. However, to the contrary, although the fuel gas and the oxidizing gas need be separated in a gas-tightly sealed manner so that they may not be mixed, it is difficult to gas-tightly seal the cell units at high temperatures. For example, the gas-tight sealing member such as a gasket or organic resin can withstand only temperatures up to 500.degree. C.
Recently, sealing means has been proposed with respect to the planar cell unit (Japanese patent application Laid-open No. 2-278,664). According to this publication, planar cell units and gas-separating plates are alternatively laminated, a reserviour is formed around the outer periphery of each cell unit or the entire outer periphery of the laminate, and sealing is effected by filing a molten glass in this reserviour. However, this method has the problem in that when the temperature is repeatedly raised or lowered, the molten glass is repeatedly solidified and melted. Therefore, the sealant has poor durability.
On the other hand, in the above cylindrical cell manufactured by Westinghouse Electric Corporation (General Energy Engineerings, 13-2, 1990), one end of the cylindrical cell unit must be closed. It is difficult from the standpoint of productivity that one end of the cylindrical cell unit is closed with a ceramic material, and suitable strength is given to this sealed portion, although the cylindrical cell unit having opposite ends opened can be relatively easily shaped by extrusion at high productivity. Furthermore, in the case of the power generator using the one-end closed cell units, oxidizing gas-introducing pipes made of a heat-resistive material need be inserted and fixed into inner spaces of the respective cell units. Owing to this, the number of constituent parts largely increases, so that working to insert the oxidizing gas-introducing pipes into respective cell units is great to make the mass production difficult.